Solar arrays and other photovoltaic (PV) devices using PV enhancement films for trapping light

ABSTRACT

A solar energy conversion assembly for efficiently capturing solar energy by providing additional chances to absorb reflected sunlight. The assembly includes one or more solar cells that each include a light-receiving surface. A fraction of light incident upon the light-receiving surface is reflected. The assembly includes a photovoltaic (PV) enhancement film of transparent material such as plastic positioned to cover at least a portion of the light-receiving surface. The PV enhancement film includes a substrate positioned proximate to or abutting the light-receiving surface. The film includes a plurality of total internal reflection (TIR) elements on the substrate opposite the light-receiving surface. The TIR elements transmit initially received or incident light to the light-receiving surface of the solar cell without significant focusing and then use TIR to trap a substantial portion of the reflected light to provide additional chances for absorption such that typically lost light may be converted to electricity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to devices and systems forconverting solar energy into electricity such as devices using thephotovoltaic effect to convert solar energy directly into electricity,and, more particularly, to a PV device such as a module, array, or panelof solar cells that includes a PV enhancement (or light trapping) layeror film to better trap or capture light or rays from the Sun that areincident on a front or receiving surface of the PV device to achieveenhanced absorption and conversion of the solar energy into electricity.The PV enhancement film is adapted to reduce reflection or loss ofincident light when the Sun is normal to the front surface of a solarcell (or to the solar module, array, or panel) and also at obliqueangles, thus reducing the need to track the Sun's position with the PVdevice including the PV enhancement film.

2. Relevant Background

With the growing interest in renewable energy including use of solarpower, there is an increasing demand for more efficient solar cells.Solar cells or photovoltaic (PV) cells are devices that convert solarenergy into electricity by the photovoltaic effect, and solar cells arewidely used in devices ranging from satellites and other applicationsincluding portable consumer electronic devices that are remote from aconventional power source. More recently, large solar power collectionsystems with arrays of cells or PV modules (or solar panels) are beingused to supply power to electrical grids for distribution to consumers.Several concerns are limiting the implementation of solar cellsincluding cost of materials and manufacturing, environmental concernswith materials such as lead, and low efficiency of the cells. As aresult, researchers continue to look for ways to lower manufacturingcosts and for ways to enhance the efficiencies of solar cells andpanels, modules, or arrays that include such cells. For example,existing solar cells, such as those based on a silicon substrate,typically have efficiencies of 10 to 20 percent, and, as a result, evensmall increases (e.g., of one to several percent) in efficiency mayrepresent large relative gains in being able to convert solar energyinto useful electricity (e.g., an increase in efficiency of 1 to 2percent represents a 5 to 20 percent or higher gain in efficiency for acell design). Even with these limitations, though, the manufacture andinstallation of solar cells and PV arrays has expanded dramatically inrecent years. Some estimates indicate that PV production has beendoubling every two years making it the world's fastest growing energytechnology, with about 90 percent of the capacity being grid-tiedelectrical systems that are ground mounted (e.g., in solar farms) or onbuilding roofs/walls. Concerns with global warming and financialincentives provided by world governments will likely only increase thedemand for PV arrays and the demand for more efficient solar cells.

Solar cells or photovoltaics (or PV devices) convert sunlight directlyinto electricity and generally are made of semiconducting materialssimilar to those used in computer chips. When sunlight is absorbed bythese materials, the solar energy frees electrons loose from theiratoms, which allows the electrons to flow through the material toproduce electricity. The process of converting light (i.e., photons) toelectricity is called the photovoltaic (PV) effect. In practice, solarcells are typically combined into modules that hold numerous cells(e.g., up to 40 or more cells, and a set of these modules (e.g., up to10 or more) are mounted in PV arrays or solar panels that can measure upto several meters or more per side, with each cell typically only beingup to 100 to 150 square centimeters in size. These flat-plate PV arraysare mounted at a fixed angle facing the Sun (e.g., south) or they may bemounted on a tracking device that follows the position of the Sun toallow them to better capture the Sun's light throughout the day. Solarcells may be formed using thin film technologies to use layers ofsemiconductor materials that are only a few micrometers thick.

The performance of a solar cell is measured in terms of its efficiencyat turning sunlight into electricity. Only sunlight of certain energieswill work efficiently to create electricity and much of this desirableenergy sunlight may be reflected from the surfaces of the solar cell orabsorbed by its materials without creating electricity. Due to these andother areas for loss of sunlight, a typical solar cell may have anefficiency ranging from about 5 to 20 percent, with some highlyefficient solar cells claiming efficiencies of up to 22 percent orhigher. For example, an energy conversion efficiency of 22 percent wasannounced in 2007 as being a world record event for a practical-sized(e.g., about 100 cm²), crystalline silicon-type solar cell (e.g., an HITsolar cell composed of a single thin crystalline silicon wafersurrounded by ultra-thin amorphous silicon layers). Low efficienciesmean that larger arrays are required to provide a particular amount ofelectricity, which increases the material and manufacturing costs. As aresult, improving solar cell efficiencies while holding down the costper cell continues to be an important goal of the solar power industry.

Even when a PV array is positioned normal to the Sun's rays, sunlight isreflected or lost to a typical solar cell, with estimates at 5 to 33percent or more of the light being spectrally reflected and lost. Insome cases, the solar cells may be protected from the environment with aglass sheet or with transparent plastic packaging. Significant effortshave been made in the solar power industry to boost efficiency byreducing reflection when the sunlight is incident at a normal angle (orwhen the angle of incidence is at or near zero degrees) on the PV array.Typical solutions have called for application of an antireflectioncoating of a material such as a SiN_(x) layer or the like on the celland/or upon the protective glass/plastic layer to minimize thereflection of light from this protective layer. The AR coating providesa layer or layers of material with a desirable refractive index andthickness (e.g., a quarter wavelength) to lessen reflection of sunlightat the coated surface (e.g., the planar surface of the sheet ofprotective glass). In some embodiments, the AR coating may be a metalfluoride combined with silica (e.g., a flouropolymer), a zinc or othermetal oxide (or other transparent conductive oxide), or other materiallayer. Recently, coatings formed of a single-layer of porous siliconoxide have been applied to a protective glass layer to decrease glassreflection by about 3 percent at noon and by about 6 percent in morningsand evenings (e.g., when the sunlight strikes the solar cell at anoblique or non-zero incident angle). Additionally, other AR coatings arebeing developed including textured-dielectric coatings andmulti-layered, nanostructured coatings (e.g., seven layers of silicondioxide and titanium dioxide nanorods). While providing someimprovements in light capture and efficiency of solar cells, existing ARcoatings generally are most useful in controlling reflection of sunlightwhen the sunlight is striking the PV array at normal (e.g., near noonfor many arrays) and do little to limit other types of reflection orbounce back losses.

As discussed above, the largest problem with the rapidly developingtechnology of photovoltaics revolves around cost-versus-efficiency.Whether traditional silicon materials are used or newer cadmiumtelluride or copper photovoltaic constructions are used, efficienciesare still a significant limitation on solar cell use and adoption byconsumers and the power industry. While multiple-layered more expensivePV cells that are designed for concentrator systems or to absorb a widervariety of wavelengths (including the longer wave lengths) aresignificantly more efficient, the more elaborate and expensivemulti-layer PV materials also result in increased ray loss at anglesother than nearly perfect toward the Sun in both axis. This additionalloss is caused by increased ray deflection as a result of cosine falloff as well as the narrow acceptance angles required by the complexityof the PV structures as they attempt to use more of the available wavelengths.

Losses in absorption into PV materials are caused by several factors.One of the factors is normal “cosine fall off” or the lack of absorptiondue to the incoming angle of the sunlight and the relationship of theincoming rays to the structures in the PV materials. For example, atypical solar cell will have an upper or front surface that is notperfectly planar but is instead textured or rough. This results in manyof the rays simply being deflected off of the surface of the PVmaterials and not being absorbed. However, this phenomenon also happenseven when sunlight or rays are directed directly into the PV materialsat zero degrees or at a perfect angle into the PV material. Part of thereason for the deflection of the rays is that the structures within thePV materials are not flat and are for the most part three-dimensional.Incoming rays, even when perfectly aimed, are bouncing off thesestructures (e.g., are reflected) and are never absorbed by the solarcell.

Hence, there remains a need for improved solar cell devices/productssuch as PV arrays that better control reflection and/or increase theamount of incident sunlight that is absorbed by the solar cells.Preferably, the improved solar cells would have improved energyconversion efficiencies with or without tracking of the Sun's position,and the solar cells and PV arrays of solar cells would not besignificantly more expensive to manufacture or requireredesigns/modification of the underlying solar cell configuration ormakeup.

SUMMARY OF THE INVENTION

The present invention addresses the above problems by providingphotovoltaic (PV) enhancement films for use with solar panels/arrays andsolar cells (or other PV devices) to increase the efficiency of the PVmaterials in absorbing available solar energy. Specifically, the PVenhancement films are designed to capture or trap light reflected fromlight-receiving surfaces of solar cells that normally would be lost anddirect this reflected light back onto the light-receiving surfaces(often at a different incidence angle) for second or additional chancesfor absorption and conversion into electricity. The trapping orredirection of the light is achieved through the use of total internalreflection (TIR), and the PV enhancement films act to pass most incidentlight onto the light-receiving surface (without significant focusing)and then trap a significant portion of reflected light back to thelight-receiving surface. The PV enhancement films may be formed of aplastic, glass, ceramic, or other substantially transparent material(e.g., highly light transmissive material such as an energy-curedpolymer) and include a thin substrate upon which a plurality of TIRelements or structures are formed (or provided).

Each of the TIR elements is designed to receive and transmit thereceived light but then to direct reflected light from the solar cellback to its light-receiving surface for possible absorption. Forexample, the TIR elements are elongated structures that are providedwith a sawtooth pattern on a side of the substrate with the triangularcross section TIR elements each providing two facets to trap light viaTIR. In other cases, the TIR elements are three-dimensional (3D)structures on the surface of the substrate that each act to reflect backotherwise lost sunlight, e.g., a plurality of hemispherical/dome shapedbodies, full or truncated cone shaped bodies (frustoconical shapes),three-sided or four-sided (or more-sided) pyramids (coming to a truepoint/apex, to a flat/planar side/facet, to a curved, domed, orhemispherical point, or other termination), and so on. Through modelingwith ray tracing computer programs, it is likely that the PV enhancementfilms will capture enough reflected light to significantly increaseefficiencies of solar cells, solar arrays, and the like. For example,modeling indicates that use of some embodiments of the PV enhancementfilms will increase efficiencies up to 10 percent while otherimplementations may increase efficiencies up to 25 percent or more (suchas in solar arrays with no tracking and with larger reflection lossesduring off-peak hours).

More particularly, one embodiment provides a solar cell assembly formore efficiently capturing solar energy by providing additional chancesto absorb reflected sunlight. The assembly includes a solar cell (notein a solar array assembly a plurality of solar cells would be includedin the assembly). The solar cell includes a light-receiving surface, anda fraction or percentage of light incident upon the light-receivingsurface is reflected over a predefined range of incidence angles (e.g.,most light may be absorbed in the range of −10 to 10 degrees but over atmore oblique angles of incidence such as greater than 10 degrees andless than −10 degrees greater and greater portions of the light strikingthe solar cell may be reflected). Significantly, the solar cell assemblyincludes a PV enhancement film formed of substantially transparentmaterial (such as a light-transmissive plastic) positioned to cover orabut at least a portion of the light-receiving surface (and, moretypically, to cover most or all of the light-receiving surface). The PVenhancement film includes a substrate positioned proximate to orabutting (except for an optional adhesive layer affixing the film to thesolar cell) the light-receiving surface. The film also includes aplurality of total internal reflection (TIR) elements on the substrateopposite the light-receiving surface. Each of the TIR elements isdesigned to direct at least a portion of the reflected light back towardthe light-receiving surface of the solar cell.

The TIR elements transmit the light incident (or most of this receivedlight) to the light-receiving surface (e.g., do not block or focus thislight) and also direct the portion of the reflected light back to thelight-receiving surface using TIR as the reflected light strikes one ormore sides of the TIR elements. Each of the TIR elements may have alinear or elongated body with at least two inward angled sides. Forexample, the TIR elements may have triangular cross sections (e.g., asviewed on end) with sides angled inward at angles of less than about 60degrees (e.g., of less than about 45 degrees). The TIR elements may beof like size and shape, or the TIR elements may have differing sizes(and/or shapes) such that alternating ones have differing thicknessesfor example (or alternating sets of the TIR elements may be configureddifferently) to tune the optical effect achieved (i.e., differing TIRelements may provide better TIR trapping effects at differing incidenceangles and it may be useful to select two or more of such TIR elementsin a single PV enhancement film). In another example, the TIR elementsare linear but include three, four, or more facets/sides such as twoinward angled sides with an upper planar side positioned therebetweenthat is parallel to the light-receiving surface (e.g., similar to across of a pyramid structure). In other embodiments, the PV enhancementfilm may include a light receiving and trapping surface that defines theTIR elements, and this surface may have a sinusoidal shape when viewedalong an edge of the PV enhancement film (e.g., to provide elongated TIRelements with a sinusoidal sectional shape). In other embodiments, theTIR elements are provided on a surface of the substrate opposite thelight-receiving surface and have 3D bodies with a shape selected toprovide a desired TIR effect (such as a plurality of pyramids or 3 to 4or more-sided pyramid shapes protruding from a substrate or sheet,numerous hemispherical shaped bodies or domes, a number of cones ortruncated cones, and the like). The PV enhancement film may berelatively thin and the TIR elements small to achieve the TIR trappingeffect, and, in one embodiment, the TIR elements each has a base and aheight less than about 10 mils in magnitude.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block illustration of a solar energy conversionsystem of an embodiment of the invention showing generally use of a PVenhancement film(s) on a solar panel or array, with the PV enhancementfilm taking any of the forms described herein to reduce reflection orbounce back of sunlight or the Sun's rays via TIR techniques;

FIG. 2 illustrates a solar cell with an imperfect front or receivingsurface and bounce back and/or reflection of incident solar radiation orsunlight;

FIGS. 3A-3C illustrate schematically directing light with a conventionalfunnel, a conventional funnel with mirrored walls, and an invertedfunnel (or truncated cone) showing that light entering a base or widerportion of a funnel has a tendency to be trapped or be directed backtoward the base of the funnel (e.g., of the optical microstructure thatmay be utilized for its TIR properties);

FIG. 4 illustrates a perspective view of a solar cell assembly includinga PV enhancement film or layer in accordance with one embodiment of theinvention with linear or elongated TIR structures or elements;

FIG. 5 illustrates an enlarged or detailed partial end view of the solarcell assembly of FIG. 4 showing in more detail a single TIR structure orelement during use to trap rays or sunlight reflected or bounced backfrom a solar cell receiving or front surface to enhance rate ofabsorption of available solar energy by the solar cell;

FIG. 6 illustrates a partial end view of a solar cell assembly similarto that shown in FIG. 5 showing exemplary details of solar cell that maybe modified to include a PV enhancement film in accordance with anembodiment of the invention with two or more flat sides or facets totrap incident light that is reflected or bounced back from other layersof the solar cell assembly;

FIG. 7 illustrates a perspective view of an additional solar cellassembly in accordance with an embodiment of the invention including aPV enhancement film placed over the top contact, which in turn isdeposited upon rough surface of PV structure;

FIG. 8 is a perspective view of a PV enhancement film showing anembodiment using a plurality of truncated cone-shaped TIR structures;

FIG. 9 is a sectional view of a single TIR structure of the PVenhancement film of FIG. 8;

FIG. 10 is a perspective view of an additional PV enhancement filmshowing an embodiment using a plurality of dome or hemisphere-shaped TIRstructures/elements to redirect reflected sunlight back toward a solarcell or PV device;

FIG. 11 is a sectional view of a single TIR structure of the PVenhancement film of FIG. 10;

FIGS. 12 and 13 are graphs illustrating reflection as a function of theangle of light in exemplary mediums;

FIG. 14 illustrates a ray tracing for a two-sided or faceted TIRstructure of an embodiment of the invention as may be used in asaw-tooth PV enhancement film as shown in FIG. 4, for example, or atextured film as shown in FIGS. 8-11;

FIGS. 15 and 16 are graphs showing light intensity as a function ofangle of incidence of sunlight upon a light-receiving surface with andwithout a covering or paired PV enhancement film with 95 percent andwith 50 percent reflectivity, respectively;

FIG. 17 is a ray tracing for a portion of a PV enhancement film inaccordance with an embodiment of the invention using 2-sided or facetedTIR structures and showing refraction effect during use of PVenhancement film;

FIG. 18 is a graph showing light intensity as a function of angle ofincidence of sunlight upon the PV enhancement film of FIG. 17;

FIG. 19 is a graph showing light intensity as a function of angle ofincidence of sunlight upon the PV enhancement film of FIG. 7;

FIG. 20 is a schematic end view of a solar array assembly showing use ofa PV enhancement film to improve efficiency of a set of solar cells ofthe array, with the film including a plurality of linear or elongatedTIR structures defined by a sinusoidal light receiving surface;

FIG. 21 is a graph showing light intensity on an array light-receivingsurface as a function of angle of incidence of sunlight upon the PVenhancement film of FIG. 20 and upon the array without the film;

FIG. 22 is a schematic end view of a solar cell assembly showing use ofa “cusp up” PV enhancement film in accordance with an embodiment of theinvention;

FIG. 23 is a schematic end view of a solar array assembly including a PVenhancement film with TIR structures with two designs that arealternated across the PV enhancement light-receiving/trapping surface;and

FIG. 24 a graph showing light intensity on a solar array light-receivingsurface as a function of angle of incidence of sunlight upon the PVenhancement film of FIG. 23 and the array without the film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, the present invention is directed to photovoltaic (PV)enhancement films for use in solar panels, arrays, or modules (or withindividual solar cells or other PV device) to improve solarefficiencies. The PV enhancement films are adapted to be applied toexisting solar arrays, panels, or modules and/or used with solarcells/PV devices during initial manufacture. The PV enhancement filmseach include a substrate of plastic, glass, or ceramic (e.g., asubstantially transparent material such as a thin polymeric film or thelike) with a plurality of total internal reflection (TIR) structures orelements on one surface. The PV enhancement film may be attached to asolar array such that TIR structure-side faces the Sun and the opposite(typically planar) side abuts the light receiving side and/or surfacesof the solar cells in the solar array. The TIR structures are eachadapted to create TIR directionally toward the photovoltaic structuresof the solar cells to enhance performance of the solar array (orindividual PV device) by deflecting unabsorbed photons back toward thephotovoltaic structures for a second (or additional) chance forabsorption and conversion into electricity. Briefly, each TIR structureon the PV enhancement film acts to allow incident sunlight to passrelatively unhindered to the solar cells of the array, and, then, totrap a substantial fraction of the reflected or bounced back light withTIR causing the initially unabsorbed sunlight or rays to be returned atleast one more time to the solar cell (with some rays being directed tothe solar cell 2 to 5 times or more before they are either absorbed orescape the PV enhancement film and the solar array that includes such afilm).

The shape of the TIR structure may vary widely to practice the inventionwith some TIR structures being linear with a triangular, a polygonal, anarcuate or other cross section while others are non-linear with avariety of shapes such as pyramids (three, four, or more angled sideswith/without an apex (e.g., a flat or shaped peak rather than a point),domes, cones, frustoconical shapes, and hexagonal shapes (e.g., texturedsurfaces with domes or other side-by-side TIR elements on a surface of asheet or film of transparent material). The angles defined by thelight-receiving/trapping surfaces of the TIR structures may also varywidely from about −5 to +5 degrees with angles in the −45 to 45 degreerange being desirable in some cases. Also, the size of the TIRstructures may be varied to practice the invention with some embodimentsutilizing TIR elements as small as about 0.25 mils (5 microns) in widthwhile others use TIR elements up to 1 inch or more in width. The height(or thickness) may also vary again with some embodiments being very thin(e.g., about 0.25 mils or 5 microns in height) while others arerelatively thick (e.g., up to about 1 inch or more in height). In oneexemplary embodiment, a two or three-faceted TIR structure is utilizedthat is about 2 mils high and about 1 mil wide at the top planar facetor side of the structure (with the base width or pitch varying with theinward angle of the side(s) of the structure).

FIG. 1 is a functional block or schematic view of a solar energyconversion system 100 in accordance with an embodiment of the invention.As shown, a light or energy source 102, such as the Sun, transmitsenergy, rays, or sunlight 104 (often termed “solar energy” or “solarpower” made up of nearly parallel light rays due to the distance betweenthe Sun 102 and the array 112) onto a solar energy collection assembly110. The assembly 110 includes a solar array or panel 112 that generallywould include numerous solar cells (not shown in FIG. 1) that each areformed of PV structures designed to convert solar energy 104 intoelectricity 120. The assembly 110 may be rigidly mounted to generallyface the Sun 102 such as on, in the northern hemisphere, a southernexposure portion of a building roof or in a solar farm, or the assembly110 may include tracking components 118 that move and position the solararray/panel 112 to follow the Sun 102 such that the sunlight 104 isalways or more often orthogonal to the receiving surface of the array112 (and included solar cells).

Electricity 120 generated by the solar cells or PV structures of thesolar array 112 may be provided to a variety of end use devices such ascharging electronics 130 for storage in a battery or batteries 132, apower grid 140 (e.g., for use by a utility to meet residential andcommercial power demands), and/or other electrical loads 150. One of thelargest problems with the use of PV arrays 112 is efficiency inconverting the solar energy 104 into electricity that can be used by theend users (battery 132, power grid 140, and/or electrical load 150).Some estimate that existing solar arrays may only be 10 percentefficient in generating the electricity 120, e.g., generate 100 W/m²from the received solar energy 104 of 1 kW/m². There are alsoinefficiencies and line losses in the transmittance and storage of theelectricity 120 that may result in an end-to-end efficiency of even lessthan 10 percent (e.g., 7 to 8 percent by some industry estimates).

The present invention is directed toward increasing the efficiency of PVdevices such as the solar array 112, and the inventors recognize that asignificant portion of the solar energy 104 incident upon a typicalsolar array 112 may be lost as shown at 106 due to reflection or bounceback. For example, losses in absorption of PV materials in the solararray 112 may be caused by normal cosine fall off or the lack ofabsorption due to the incoming angle of the sunlight 104 with relationto the structures in the PV materials. Even when tracking 118 isutilized, a portion 106 of the sunlight 104 is lost due to reflection(such as off of a protective glass surface, due to surfaceirregularities, and so on).

For example, FIG. 2 illustrates a solar cell 210 that may be includedwithin the array 112, and, due to manufacturing techniques (or even byintentional design) the light-receiving surface or front side 212 of thecell 210 may be irregular or be textured (e.g., not planar). As aresult, sunlight 220 that strikes the cell surface 212 even at a zerodegree angle of incidence may be deflected as shown at 224, 225, 226,227 while other rays or photons 222 are absorbed by the PV materials orstructures of the cell 210 and converted into electricity. The problemwith deflection of rays 224, 225, 226, 227 (or rays 106 in FIG. 1) isworsened when no tracking is provided such that the rays 104, 220 strikethe surface 212 (or solar array 112) at oblique angles (e.g., anglesless than or greater than normal or zero degrees as measured from normalto the surface 212 or light-receiving surface(s) of array 112 or itssolar cells).

With reference again to FIG. 1, one solution to improve the efficiencyof the collection assembly 110 (or of array 112) is to bounce at least aportion or fraction of the unabsorbed rays 104 back into/onto the PVmaterials (or solar cells) of the solar array 112, which reduces theamount of solar energy 106 that is lost from the system 100. To thisend, a PV enhancement film 114 is positioned over all or a subset of thesolar cells or PV material of the array 112 to trap at least a fractionor portion of light reflected or deflected off of the surfaces of thearray 112 without being absorbed and redirect it back at least once ontothe array 112 to increase chances of absorption and generation ofelectricity 120. The film 114 includes a plurality of TIR structures orelements on the surface receiving the sunlight 104, and each of thesestructures or elements is adapted to make use of TIR techniques to allowincoming rays 104 to enter and strike the solar cells/PV materials ofarray 112 while also promoting or enhancing the possibility of raysbouncing back onto the solar cells/PV materials if not absorbed (e.g.,if reflected/deflected off light-receiving surfaces/structures in array112). As will be explained more below, each TIR structure on film 114 isa shape formed on the light-receiving surface of the film 114 in orderto provide TIR of reflected or deflected rays. In essence, the TIRstructures are “reverse funnels” that allow rays to enter but act todeflect rays back to the PV materials of the array 112 by design asthose reflected/deflected rays try to exit, as lost rays 106, away fromthe PV material at certain angles. The TIR structures of film 114continue to deflect a portion or fraction of the unabsorbed rays orsunlight such that the trapped and/or deflected rays within the film 114are directed back to the PV materials, thereby increasing efficienciesof the materials and array 112 from about 5 to 20 percent up to 10 to 25percent or more in some cases.

The film 114 may be retrofitted onto an existing solar array 112 orcombined with new PV materials or solar cells during initial fabricationof the array 112, with either fabrication method of the array beingperformed relatively inexpensively (e.g., a plastic film 114 withnumerous TIR structures may be attached with adhesives or other methodsin a few minutes even after the array 112 is in position in a solarfarm/plantation or a rooftop setting or may be laminated or otherwiseapplied over the solar cells of an array or over each solar cell as itis formed (e.g., the film 114 may include a plurality of segments orindividual films each applied to a solar cell to form a solar cellassembly that may then be included within the array 112). The film 114(or films as may useful to provide desired coverage of a larger array112) may be made of a wide variety of materials such as readilyavailable plastics that are transmissive to sunlight 104 such as apolymeric sheet or the like or made of glass or certain ceramics thatare transparent or at least highly transmissive. In some embodiments,the materials used for forming the film 114 and its TIRstructures/elements are chosen from glass, nearly any type of clear(i.e., transparent to translucent/less transmissive) plastic includingbut not limited to PET, propylene, OPP, PVC, APET, acrylic, or any clearplastic, and/or a ceramic. In many embodiments, the preferred basematerial is a plastic dues to its durability, low material andmanufacturing costs, and light weight, and the plastic of the film 114may be extruded, calendared, cast, or molded to provide the functionalTIR structures described herein.

The TIR structures includes on the PV enhancement films of embodimentsof the invention function to create TIR on the light-receiving surfacesof solar cells/PV materials, and the TIR structures may be described as“reverse funnels” for directing light. FIGS. 3A-3C illustrate aconventional funnel or cone structure 300, an inverted funnel or conestructure 320 (e.g., with its larger portion used as its lower or baseportion), and a funnel or cone structure with mirrored walls. Indescribing attempts to direct or herd light rays, the rays of light donot react or behave as initial logic may suggest such as if oneattempted to treat light as a fluid and apply fluid dynamics principles.In fact, light may be thought of as acting in an opposite fashion as aliquid in a funnel. In other words, when creating a funnel for fluids,the fluid will behave under the force of gravity in a predictable mannerwith gravity pulling the fluid from a larger end of the funnel to thesmaller portion and the exit (e.g., funneling oil into a vehicle or thelike with the exit having a smaller diameter). In contrast, whenstructures are created with mirrors, glass, or other reflectivematerials with narrowing pathways, the opposite effect happens whenlight is directed into the light “funnel.”

FIG. 3A shows a funnel structure 300 in a material with an index ofrefraction of 1.49 and non-reflective walls. The incoming rays 312 arereflected as rays 314 that undergo smaller angles of reflection untilthe rays 316 finally escape from the surface or sidewall 310 of thecone/funnel structure 300. In contrast, as shown in FIG. 3B, an invertedfunnel structure 320 allows received or incident light 332 to enter andcontinue on or be focused as shown at 334 and 336 through the largeropening of structure 320 (e.g., a ray entering a cone of a material withan index of refraction of 1.49 undergoing TIR). FIG. 3C illustrates forexample of a cone structure 350 with reflective or mirrored sidewalls352 and an index of refraction of 1.49 arranged with a narrowingdiameter (e.g., a truncated or frustoconical shaped structure 300). Anattempt to funnel incoming or received light rays 354 results in thelight actually backing out as shown at 354 of the funnel structure 356by being deflected/reflected off the narrowing walls 352 (e.g., the raysare reflected off the walls 352 at angles that sequentially differ suchthat they get reflected back out of the cone 350 such that rays thatwould normally escape from the funnel tip are reflected back up thefunnel). The inventors recognized that the structure (or other “funnel”structures) 350 could be utilized to trap unabsorbed sunlight within asolar array or solar cell when the ray 354 is considered a photon or rayof sunlight that is being deflected or reflected from the cell or PVmaterials. In other words, the TIR elements/structures are arranged asinverted funnels as shown in FIG. 3B (or inverted versions of thoseshown in FIGS. 3A and 3C) upon the solar cells (or arrays) such thatincident light is allowed to enter and be focused upon a solar cell/PVmaterial while unabsorbed light is deflected back for a second (or thirdor more) chance for absorption.

The concept of using TIR in thin films (e.g., plastic films with asubstrate with TIR structures on one side that are applied over solarcells) is believed to be a new concept. In order to use TIR as a methodof turning rays or limiting loss of sunlight from a solar cell/PVmaterial, it may be useful to describe basic physics or geometry of theangles and the resulting behavior of the rays. Sunlight or rays mayenter into a transmissive TIR structure or element with refractiveindexes of between 1.1 and about 2.0, with normal ideals between 1.4 and1.65. Once rays enter the TIR structures, the rays will not exit the TIRstructure if they remain within 42 degrees of parallel to the wall ofthe structure when striking the wall of the structure. Rays within thisTIR physics law will deflect inside the TIR structure until they find anescape or are absorbed by the PV materials/solar cell and are convertedinto electricity. The “escape” out of the TIR structure would typicallybe the result of a deflection of past 42 degrees as measured from a wallof the TIR structure (e.g., an internal “surface” of a wall or facet ofa TIR structure that comprises a solid body of plastic, glass, ceramic,or other highly transmissive material), creating a more direct “strike”of the surface allowing the rays to exit.

One idea supporting the invention is that many losses and decreases inefficiency in various PV films are the result of rays not being absorbedinto the PV material, e.g., absorption of photons to start thephotovoltaic effect (i.e., process of the electron moving down (creatinga hole) to be replaced by an electron of the opposite polarity andtherefore creating a flow of electrons to establish a current). Instead,the rays bounce off the three-dimensional (3D) surface ornon-planar/textured surface of the PV materials or solar cell, whetherthe film is using silicate, cadmium telluride or other technology ormaterials. Since the goal of a solar cell design is for the rays toenter the PV film, and, therefore, the structures, PV films can be madeof many layers, targeting the various wavelengths of the incidentsunlight. Since a large portion of the solar energy lies in the shorterwavelengths for the PV process, these wavelengths are specificallytargeted either on one or more layers for absorption. PV materials areoften made of a pure material such as silicon mixed with tiny amounts ofcontrolled impurities to produce controlled energy levels or bands inthe material. Depending on the materials and amounts used there arevalence bands and conduction bands. The photons coming from a source oflight will cause electrons to occupy conduction bands that can conductelectrons to an electrode. On the other side of the material, there areholes that are effectively positive charges that were produced by thephotons that traveled to the electrode on the other side of the PVmaterial to complete the electric circuit. It is not the purpose here todwell on these processes that are covered in solid state physicstextbooks and available reference materials but to generally say thatwhen photons are absorbed under the right conditions an electricaloutput will be generated.

One of the main problems with the ray absorption into PV materials isthat incoming rays tend to bounce off the surface of the PV materials atextreme angles, significantly decreasing the ray's chance of absorption.This is attributed generally to a phenomenon known as cosine fall off.In this case, the incoming rays are coming in from difficult angles inthe mornings, evenings, and/or differing seasons (or a combinationthereof) that allow an undesirably small fraction or portion of the raysto be absorbed by the solar cell or array containing numerous solarcells. In addition, improper orientation of panels/panels with regard tothe azimuth of the Sun may also contribute to or lead to loss ofavailable solar energy.

It is important to note, however, that even with perfect tracking of thesun with PV materials, the angles of the structures themselves create arandom-like mathematical percentage of rays bouncing off structureswithin the PV film and not being absorbed. Even short wave length rayscoming in directly may hit structures or textured surfaces of a solarcell at angles that do not allow absorption (as shown in FIG. 2). Whilecertain types of coatings (e.g., AR coatings) can help improveefficiencies by reducing reflection in some cases, the nature of thepresent invention differs as it provides a method that makes use of TIRto improve solar energy conversion efficiencies of solar cells and otherPV devices (and of solar panels/arrays incorporating such solar cellsand PV enhancement films). The PV enhancement films include TIRstructures/elements that allow the incoming or incident rays to enterunencumbered through the micro-structured film (e.g., a sheet ofplastic, glass, or ceramic with TIR elements on one side without shading(such as is the case with the micro-mirror systems). Then, if a ray isreflected from the surface of the PV structures, each TIR structure isdesigned with an ability to create TIR, e.g., based on the angle ofdeflection parallel to the pathway of exit of that ray (which must beless than 42 degrees as measured from the wall of the TIR structure uponwhich the ray strikes). The trapped or deflected ray continues to“bounce” off of the internal structure of the TIR element at least once,possibly two, three, or more times based upon the shape of the structureand to “strike” the PV surface again (at least a second time) forpossible absorption.

In fact, this TIR effect may happen a few times up to dozens of timesuntil the ray is absorbed into the PV material generating energy orexits the structure because of a deflection creating an angle thatexceeds 42 degrees (a more direct “hit”) to the wall of the structure.Therefore, use of a PV enhancement film with TIR elements/structureswith a solar cell/PV device in accordance of invention does not insurethe absorption of each ray, but it does increase the odds of absorptionby giving a portion or fraction of the unabsorbed rays another chance oropportunity at absorption by bouncing the ray back to the PV material ata different angle (at least the probability is that the ray will beincident upon the solar cell at a different angle than when firstreceived). In many cases, a ray, if rejected again by the PV materialwith a deflection, will in theory have dozens more chances ofabsorption. Ultimately, the ray will either be absorbed or will find anexit angle from the structure that exceeds 42 degrees from parallel ofthe surface of the wall of exit. In ray tracing programs run by theinventors to test use of TIR structures/elements of the invention, usinga 95% reflective mirrored surface in place of a light-receiving surfaceof a solar cell shows that rays may bounce dozens of times within thestructures before exiting the structure. In other words, the TIRelements or microstructures can increase a deflected ray's odds of beingabsorbed by a significant amount or by many times in this TIRlight-trapping process.

FIG. 4 illustrates a solar cell (or PV device) assembly 400 inaccordance with one embodiment of the invention. As shown, the assembly400 includes a solar cell or PV device 410 (such as a silicon-basedsolar cell, a thin-film device, a GaAs/Ge solar cell, and nearly anyother PV design) with a rear or back surface (e.g., a back contact) 412and a front or light-receiving surface 414 (e.g., a front contactcovering PV materials such as p-semiconductor layer as may be found in asilicon solar cell/wafer design, an AR coating, a glass or otherprotective layer, or other surface directed toward the Sun in a solarcell design). Since a significant portion of incident light upon thelight-receiving surface 414 of the solar cell may be reflected,deflected, or otherwise unabsorbed, the assembly 400 also includes a PVenhancement film 420. The film, sheet, or layer 420 is positioned tocover all or a portion of the light-receiving surface 414 and may beapplied or attached via an adhesive layer 418 (e.g., a substantiallytransparent or at least translucent adhesive) such that the film 420abuts or is positioned proximate to the surface 414 of cell 410. Inother embodiments, other mounting methods such as mechanical methods ordirect lamination onto the surface 414 may be used to provide the film420 on the cell 410 (and, in other cases, the film 420 may be providedas an integral layer or portion of the cell 410 such as part offormation of a protective glass, plastic, or ceramic coating on the cell410). In some cases, the PV enhancement film 420 will be manufacturedseparately from the cell 410 and applied later as a modification orretrofit (e.g., to a cell 410 already in use in a solar array or modulein a solar farm or on a roof mounting) while in some cases the PVenhancement film 420 is provided on the cell 410 as part of the originalfabrication of the cell 410 and assembly 400.

As shown, the PV enhancement film 420 includes a substrate 426 with afirst side/surface 422 that is positioned proximate to the front orlight-receiving surface 414 of the solar cell 410 (and may be considereda solar cell-facing or mating surface). The substrate 426 is generally arelatively thin layer formed of substantially transparent material(e.g., the same material as used for the TIR structures 428) and thecell-mating surface 422 often is generally planar to provide minimalinterference with light passing from the PV enhancement film 420 to thecell 410. On a second side/surface 424 of the substrate 426, the PVenhancement film 420 includes a plurality of TIR structures or elements428. The side/surface 424 may be thought of as the light-receivingsurface/side of the film 420 and is spaced apart from the cell surface414 by the thickness of the substrate 426. In this embodiment, the TIRelements 428 are two-sided or two-faceted structures with elongatedbodies extending over the length (or width) of the solar cell 410surface 414 as is shown with the length of the film, L_(PV film). TheTIR elements 428 are linear elements with a triangular cross section inthis embodiment with the two facets or sides of the structure 428forming the sides of the triangle and the base mating with or being anintegral connection with the substrate 426. The size and shape of thetriangular cross section of each TIR element 428 may be varied topractice the invention, and it may be varied depending on the intendeduse of the cell assembly 400 (e.g., 45 degree facets may be used for acell assembly 400 used in a solar array (see FIG. 1) that is used withtracking such that sunlight is typically receiving with an orthogonalorientation while smaller (or larger) facet or side angles may be usefulwhen no tracking is provided and a large portion of the sunlight strikesthe cell assembly at oblique angles). The PV enhancement film 420 may beconsidered a “sawtooth” design with the TIR elements 428 being the teethof a saw blade.

FIG. 5 illustrates an enlarged end view of the solar cell assembly 400during use to trap sunlight showing details for a single TIR element 428of the PV enhancement film 420. As shown, the PV enhancement film 420has a thickness, t_(PV film), made up of the thickness of the substrate426 and the thickness or height, H_(TIR element), of each TIR element428. The thickness of the film, t_(PV film), typically will range fromup to about 1 micron to about 0.25 inches, with the substrate 426 havinga thickness to facilitate structural integrity of the PV enhancementfilm 420 and facilitate manufacturing. Each TIR element 428 has a width,W_(TIR element), that will also vary to practice the invention and willdepend upon the facet or side angles, θ₁ and θ₂, will typically be up to1 micron to 1 inch or more. The facet-defining angles, θ₁ and θ₂, aretypically equal such that the triangular cross section of the TIRelement 428 is an isosceles triangle, but this is not required topractice the invention. In one embodiment, the facet-defining angles, θ₁and θ₂, are chosen to be less than about 45 degrees (as measured from aplane parallel to the surface 422 of substrate 426 or from a second sideof substrate 422 from which the TIR elements 428 protrude, which is alsoparallel to the surface 422), with 45 degrees being useful when receivedsolar energy is generally orthogonal to the light-receiving surface 414of the solar cell 410 and smaller angles being useful when rays arereceived at more oblique angles.

As shown in use, a first ray 520 of sunlight is received or is incidentupon a side or facet 510 of the TIR element 428 at an angle that is nearnormal or orthogonal (as measured from a plane orthogonal or normal tothe light receiving surface 414 of the solar cell 410 with the ray 521being shown at about −10 degrees). The received sunlight 521 is allowedto enter or pass through the TIR element 428 with some diffraction asshown with ray 522, and the ray 522 passes through the PV enhancementfilm 420 including the substrate 426 where it strikes the lightreceiving surface 414 (and/or PV structures) of cell 410. In thisexample, the ray 522 is not absorbed to make electricity but is insteadreflected or bounced off at a different angle as shown with ray 524. Theray 524 strikes the side or facet 510 of the TIR element 428 at anangle, a, and when this angle is less than about 42 degrees (as shown),the ray is trapped by facet 520 and reflected as shown with ray 526 backtoward the light receiving surface 414 of cell 410 (e.g., a TIR effectis achieved with TIR element 428). Again, the ray 526 may not beabsorbed by the solar cell 410 and instead reflected/bounced back asshown at 528 where it strikes the facet 520 to again betrapped/reflected at a differing angle as ray 530. Ray 530 strikes thelight receiving surface 414 of the cell 410, where it is absorbed intothe cell 410 as shown with absorbed energy/ray 532.

In this example, the TIR element 428 trapped the non-absorbed rays 524,528 using TIR and provided the solar cell 410 with two additionalchances to absorb the solar energy, which otherwise would have been lost(e.g., if conventional AR coating had been used the ray 524 would likelyhave been lost from the assembly 400). In practice, the number ofadditional chances will vary widely and will likely range from 0 to 12or more (with 0 occurring when the first reflected ray 524 strikes thefacet 520 at an angle, α, greater than 42 degrees), with a generaldescription being that the TIR element 428 acting to trap at least afraction of the reflected light and direct it back onto thelight-receiving surface 414 for one or more chances at absorption. Forexample, a cell 410 may experience up to 70 percent loss of sunlight dueto reflection and/or bounce off of received rays from thelight-receiving surface 414, and the PV enhancement film 420 with use ofthe TIR elements 428 (linear bodies with two-sided cross sectionalshapes) may be able to force 35 percent or more of the lost sunlight tostrike the light receiving surface 414 at least a second time (with somerays being trapped and forced to strike the surface 414 multiple timesby the TIR element 428).

Similarly, other rays 540 may strike the TIR element 428 at differingangles (such as at more than 20 degrees as shown) and be allowed toenter the TIR element 428 as shown with ray 542. The ray 542 passesthrough the TIR element 428 and strikes the light-receiving surface 414of the cell 410 where it is reflected or bounced off as ray 544. The ray544 strikes the facet/side 520 at an angle, β, less than about 42degrees (in this case), and is deflected as ray 546, which strikessurface 414 of cell 410 and is again reflected as ray 548. Theunabsorbed ray 548 strikes facet 510 is again trapped by TIR andreflected as ray 550 toward the light-receiving surface 414, but thisadditional chance provided by the TIR element 428 results in the ray 550being absorbed as shown at 554. It will be understood that FIG. 5represents a greatly simplified description of the use of a PVenhancement film 420 with TIR elements 428 as numerous rays would bestriking the TIR elements 428 along their lengths and across both facets510, 520, and a large portion or fraction is absorbed by the solar cell410 upon initial contact. But, FIG. 5 is useful for understanding howthe PV enhancement film 420 may be used to provide TIR structures orelements 428 over the light-receiving surface 414 of a solar cell 410 tocapture at least a fraction of the rays that are not absorbed by thecell 410 so as to increase the efficiency of the solar cell 410 byproviding the PV structures/components of the cell 410 additionalchances to absorb the rays. The “second chance” rays or trapped rayswill typically strike the surface 414 at a different location than theinitial receipt location and at a different angle, which may alsoincrease the odds that the rays will be absorbed by the cell 410.

The shape of and/or wall angles the TIR element may be altered inaccordance with the invention to provide better or differing TIR effects(or differing abilities to trap light on the solar cell/PV device frontor light-receiving surface). For example, the inventors believe it maybe beneficial to utilize TIR structures with 3 or more facets/sides totrap sunlight received over a range of angles (e.g., when tracking isnot provided for a solar array or the like). FIG. 6 illustrates an endview similar to that of FIG. 5 of a solar cell assembly 600 with a PVenhancement film 610 applied to (or positioned over/adjacent) a solarcell. In this case, the PV enhancement film 610 is formed with asubstrate 612 abutting the solar cell and supporting on an opposite sidea plurality of linear (or elongated) TIR elements 614. Each TIR element614 has a three-sided cross section, with first and second sides 616 and620 that may be angled inward to meet a top side/surface 618. The topsurface/side 618 may be planar and parallel to substrate 612 (or itsside that abuts the solar cell) and the angled sides 616, 620 may beangle angled inward at a range of angles such as up to about 15 degreesto about 45 degrees or more. Light incident upon the assembly 600 isshown at 602 as may be the case at noon or when tracking is used, andsuch orthogonal rays 602 may generally pass through the film 610 forsubstantial absorption by the cell components. In contrast, rays 603that are more oblique to the cell and substrate 612 may strike one ofthe facets 616, 618, 620 and pass through the substrate 612 and one ormore layers/components of the solar cell, with received ray 604 shownpassing through an AR coating 634, a protective glass layer 632 and anupper contact 630. At this point (or earlier), the ray 605 is reflectedor bounced back toward the PV enhancement film 610 where it strikes thesame or another facet 616, 618, 620 and is deflected as ray 606. It maybe directed toward the solar cell or to strike yet another facet 616,618, 620 as trapped/TIR-captured ray 607 for absorption upon this second(third, fourth, or later) opportunity provided by the PV enhancementfilm 610. As with other films, the substrate 612 and the TIR elements614 typically are formed of the same materials such as a plastic, aglass, and/or a ceramic that is substantially transparent (i.e.,relatively highly transmissive) to light 602 such as by extrusion orother pattern forming on a sheet or film of plastic or the like.

As will be understood, the PV enhancement films such as film 610 may beused with nearly any solar cell design and/or PV component wherereflection or loss of incident light is a concern. FIG. 6 illustrates arelatively common solar cell arrangement that may be used in a solarcell assembly 600 for exemplary purposes. The solar cell includes a backmetallic contact 670 and a front contact (e.g., a transparent conductiveoxide (TCO) electrode such as tin oxide, zinc oxide, aluminum-doped zincoxide, indium tin oxide, and the like) 630. Sandwiched between thesecell components is a thin-film solar cell layer 640 (such as asemiconductor film including one or more semiconductor layers such asamorphous silicon with n-type and p-type conductivity regions,CdS/CdTe-based solar cells, crystalline solar cells, and the like withthe particular type of solar cell layer used not being limiting to theinvention and use of a PV enhancement film), and, in this case, anoptional reflection component 660 is provided, e.g., a reflectionenhancement oxide or ethyl vinyl acetate (EVA) layer, a reflectiongrating etched on the backside of the substrate and a textured photoniccrystal as a backside reflector, or the like. Additionally, a protectiveglass 632 may be provided to protect the solar cell layer 640 and frontelectrode 630, and this is often the case in assemblies 600 in which thePV enhancement film 610 is retrofitted onto a previously built and/orinstalled solar cell (or solar array with a plurality of solar cells).An AR coating 634 (e.g., a metal fluoride and silica composite or thelike) may be included in such cases to control reflection from the glass632 (e.g., the AR coating 634 may be found in many existing solar cellsthat include a glass coating 632).

FIG. 7 illustrates another embodiment of a solar cell assembly 710 thatincludes a solar cell 720 and a PV enhancement film 730. In thisembodiment, the solar cell 720 may include a substrate 721 that mayserve as one of the cell's electrodes, with a layer 722 depositedthereon. The layer 722 may be a silicon layer or other layer useful forabsorbing solar energy. For example, the layer 722 may be formed as aSi-layer doped and otherwise formed to provide a n-type semiconductor. Ap-type semiconductor 724 may then be formed upon the n-typesemiconductor layer 722 in such a manner that results in a textured orrough surface 725 (e.g., with a plurality of pyramids or the like). Afront or counter electrode 726 is formed with or over the layer 724 toform the solar cell 720. As may be expected, the roughness or texture ofthe light-receiving surface 725 of the solar cell 720 may result in aportion of incident light being reflected or deflected and not absorbedby the layers 722, 724 to generate electricity.

To enhance the efficiency of the cell 720, the solar cell assembly 710includes a PV enhancement film 730 that is positioned over, and may bejoined to, the front electrode 726 so as to substantially or completelycover the light-receiving surface 725. The PV enhancement film 730 issimilar to the other films shown thus far in that it includes asubstrate 732 of substantially transparent material upon which aplurality of linear or elongated TIR elements 734 are formed (orotherwise provided such as by a second depositing step or the like). Thefilm 730 differs, though, as the TIR structures are formed with anarcuate light receiving/trapping surface such that each TIR structuremay be considered to have a semi-circular cross section (e.g., the bodyof each TIR element 734 may be thought of as half or less of acylinder). The TIR elements 734 are aligned with their elongated bodiesside-by-side and with their longitudinal or central axes parallel.Again, the height and width of the TIR elements 734 may be widely variedto practice the invention and to achieve a desired TIR effect (e.g.,better TIR at particular angles of incidence of received sunlight and soon to suit a particular solar cell 720). For example, each TIR elementmay be up to 0.25 inches in height but typically may be relatively thinsuch as several mils or less in height and may be up to about 1 inch inwidth but typically 10 or more TIR elements 734 are provided per inch ofwidth of the film 730.

In some embodiments, PV enhancement films are provided with numerous TIRstructures or elements formed upon a surface of a substrate and the TIRstructures do not have elongated or linear bodies. For example, FIGS. 8and 9 illustrate another embodiment of a PV enhancement film 800 thatmay be used with any of the solar cells shown herein or, more typically,with a solar array of such cells as shown in FIG. 1. The PV enhancementfilm 800 includes a substrate 810 with an upper or light receivingsurface 812 (the opposite surface of the substrate 810 typically ispositioned against or near the light-receiving surface of a solar cellor solar array/module/panel). On the substrate surface 812, numerous TIRstructures 820 are formed or provided to trap light using TIR. The TIRstructures 820 may be considered inverse funnels with a frustoconicalshape defined by an angled inward sidewall 824 that extends about thecircumference of the structure 820 and upward to a top or upper surface828. The funnel-shaped TIR structure 820 has its larger wider “opening”(with a first diameter) or its base against the surface 812 of thesubstrate 810 and angles inward at an angle, γ, such that its smaller“opening” (with a second diameter smaller than the first diameter) atsurface 828 is part of the light receiving surface. In this arrangement,as explained with reference to FIGS. 3A and 3B, incident or receivedsunlight or rays are allowed to enter the funnel-shaped TIR element 820but have more difficulty exiting and are trapped by TIR. The angle, γ,is typically less than about 60 degrees and more typically less than 45degrees, the height of the structures 820 is typically relatively smallsuch as less than about 1 inch and more typically, less than 0.25 inches(e.g., 1 to 3 mils or the like), and the diameters of the base and atsurface 828 are typically relatively small such as less than about 1inch but more typically less than 0.25 inches (e.g., less than about 5mils or the like). The TIR elements 820 may be spaced apart on surface812 of substrate 810 but more typically little or no space is providedbetween adjacent ones and/or rows of the TIR elements 820 to providemore TIR and trapping of reflected light. The TIR elements 820 functionto provide trapping of reflected light in a manner similar to that shownfor the TIR elements of FIG. 6 but in a 3D manner rather than a 2D orlinear manner.

FIGS. 10 and 11 illustrate yet another PV enhancement film 1000 inaccordance with the present invention that may be used with solar cellssuch as an additional layer or component of a solar array as shown inFIG. 1. As with the film 800, the film 1000 includes numerous,non-linear TIR elements 1020 formed upon a surface 1012 of the filmsubstrate 1010. The TIR elements 1020 in this case are shaped as smalldomes defined by a light receiving/trapping surface or wall 1024 that isshaped such that each dome 1020 has a semi-circular cross section butoverall has a hemispherical shaped body defined by a width of a base (orbase diameter), W_(Base), and a dome/body height, H_(Dome) (e.g., a basewidth of less than about 1 inch and more typically quite small such asless than 0.25 inches such as less than several mils and a dome heightthat is also relatively small such as less than about 1 inch and moretypically less than 0.25 inches such as less then several mils). Again,the TIR elements 1020 may be spaced apart but more typically are tightlyspaced upon the surface 1012 to trap a larger percentage of reflectedlight. Differing sized and shaped TIR elements may be used in a singlefilm, but to facilitate manufacturing and provide relatively consistentuse of a PV device/materials, each of the TIR elements 1020 issubstantially identical in shape and design.

The above description provides a general description of applications fora PV enhancement film with TIR structures or elements to improve theefficiencies of solar cells (or other PV devices) and of solar arrays,modules, and panels that incorporate such solar cells and PV enhancementfilms. The above description provides description of some PV enhancementfilms and TIR elements, and it also provides an introduction into howthe TIR elements trap light using TIR to direct at least a fraction orportion of reflected (unabsorbed) sunlight back onto the solar cell/PVstructures. At this point, it may be useful to provide additionalexplanation of the operation of TIR elements provided in PV enhancementfilms, and the following description provides additional embodiments ofPV enhancement films along with a more detailed explanation of theoptical principles utilized by the invention. Portions of thedescription include PV enhancement films and TIR elements shown moreschematically than in prior descriptions/figures, but it will beunderstood that such embodiments or models used for computer-simulationsand ray tracings may readily be implemented in physical products (e.g.,TIR structures or elements provided on a side of a substrate or film ofplastic, glass, ceramic, or substantially transparent material) forapplication over solar cells/solar arrays as discussed above.

This invention generally concerns the use of optical films (e.g., PVenhancement films) that make use of total internal reflection (TIR) andrefraction (e.g., TIR structures or elements that provide TIR and alsorefraction). The TIR elements redirect sunlight reflected off thesurface of a photovoltaic (PV) cell back to the cell so that energy thatwould be normally be lost for electrical power generation is used for(or at least available for) power generation, thus producing more powerthan if the cell did not have the optical or PV enhancement film as partof its assembly or covering it in a solar array. Embodiments of theinvention make use of the properties of light reflection expressed byFresnel reflection coefficients.

Relevant equations for modeling and/or evaluation of PV enhancementfilms and solar cells without such films can be written in the followingmanner: θ1=angle of incidence; θ2=angle of reflection; Rs=power in planeperpendicular to incidence; Rp=power in plane of incidence; Rs=(sin(θ2−θ1)/sin (θ2+θ1))^2; Rp=(tan(θ2−θ1)/tan(θ2+θ1))^2. A plot or graph1200 is shown in FIG. 12 of the Fresnel reflection as a function ofangle for light in a medium of index 1.00 intersecting a medium of index1.49 with lines 1210, 1220, and 1230 representing average reflection,light polarized parallel to plane, and light polarized perpendicular toplane, respectively. The curves of graph 1200 show that about 4 percentof the light is reflected at normal incidence. As shown, the averageamount of light reflected slowly increases to about 70 degrees and thenthe amount reflected increases rapidly with increasing angle. The curve1210 is the total energy reflected. In the case where the light is in amedium of index 1.49 and passes to a medium of index 1.00, FIG. 13illustrates a plot or graph of light reflection 1300 with lines 1310,1320, and 1330 representing average reflection, light polarized parallelto plane, and light polarized perpendicular to plane, respectively.

In this case, FIG. 13 shows with line 1310 that the average amount ofreflected light slowly increases up to about 35 degrees of incidence andthen very rapidly increases at about 42 degrees where all of the lightis reflected or total internal reflection occurs (TIR). One base idea ofthe invention is to have an optical material shaped so as to allow themaximum amount of energy (or a significant portion) to reach the PVmaterial of the solar cell/PV device (e.g., provide numerous TIRelements on a PV enhancement film). The reflected portion of light,which would otherwise go to waste, reflects off of the optical materialat an angle that gives TIR so that it strikes the PV material at least asecond time (and, in some cases, a third, fourth, or “N” times). Inanother case, refraction is used and will be discussed later (hence, theTIR elements/structures may use TIR, refraction, and/or other opticaltechniques to trap reflected light and/or direct it back toward the PVmaterials of a cell and are not limited to “total” internal reflection).The graphs 1200 and 1300 of FIGS. 12 and 13 show how this might be done.The amount of light lost to reflection when light enters a higher indexof material is not high in the 40 to 50-degree angles of incidenceregion but when light exits at that angle it is mostly reflected (again,remember the funnel concepts described earlier where light may bereceived by a TIR element as described here but when reflected from asolar cell's light-receiving surface it may be reflected back to thesurface for possible absorption).

For example, the TIR structure 1410 shown in FIG. 14 makes use of TIR toredirect light that has been reflected from the PV surface 1411 (whichin FIG. 14 is assumed to be 95 percent reflective or a mirrored surfacefor showing functionality of the TIR structure 1410). As describedearlier, the TIR structure 1410 may be formed of a substrate 1412 andtwo or more facets or sides 1414, 1416 (shown at angled offsets from thesubstrate in the range of 20 to 30 degrees but, of course, many otherfacet/side angles may be used to define the facets 1414, 1416 and theTIR element 1410). The TIR structure 1410 modeled by this ray tracinghad a pitch or base width of 20 mils and a thickness or height of thepeak (mating line between the facets 1414, 1416) of 2.1 mils. Themodeled TIR element 1410 may be a linear or elongated structure withFIG. 14 representing an end view or cross section or it may be astandalone or unitary component with a circular cross section along itscentral axis (passing through the peak between facets 1414, 1416) and aside sectional view as shown when a plane is passed through central axisand cylindrical body (e.g., see FIGS. 8-11 for illustration of similarunitary or non-linear TIR elements). The TIR element 1410 was assumed tobe formed of a material (such as a plastic) with a refractive index inthe range of 1.4 to 1.6 (e.g., about 1.47), and the traced rays 1420were assumed to strike the TIR element 1410 at an angles of incidence ofabout −30 degrees (as measured from normal or orthogonal to thesubstrate (and light-receiving surface 1411).

FIG. 14 shows a ray tracing in which rays have been ray traced throughthe TIR structure 1410 (e.g., when combined with other similarstructures could provide a saw tooth structure in a PV enhancement filmas shown in FIG. 4). The tracing shows rays 1420 enter the structurewith little loss but when exiting after reflecting off a PV surface 1411are reflected back to the surface 1411 for a second (or third or more)chance for absorption. For example, incident ray 1421 is shown to passinto the TIR structure 1420 after striking outer surface of facet 1414with some refraction as shown with ray 1422 traveling through TIRelement body and substrate 1412. The ray 1422 is not absorbed but isinstead reflected as ray 1423, which strikes the inner “surface” of thefacet 1414. Since this angle is less than about 42 degrees, TIR causesit to be reflected/directed as ray 1424 back to surface 1411. Upon thissecond chance/second striking of surface 1411, the PV material mayabsorb the ray 1424 or, as shown in this example, the reflection maycontinue with ray 1425 striking the inner surface of the second facet1416 and being reflected again as ray 1426 toward surface 1411. In thisexample, the ray 1421 and its energy was presented to thelight-receiving surface not just once as ray 1422 but also twoadditional times as rays 1424 and 1426, which also struck at differingangles that may further enhance the odds/chances of successfulabsorption by an operating solar cell.

Of course, the TIR element 1410 is not able to trap all rays 1420 but,instead, only a fraction or portion of the rays 1420 is redirectedtoward the surface 1411 at any particular angle of incidence. Forexample, ray 1430 strikes facet 1414 and is received as ray 1431 for itsfirst chance at absorption as it strikes light-receiving surface 1411.However, if it is not absorbed, the ray 1432 may be reflected in such away from surface 1411 that is strikes facet 1416 at an angle greaterthan about 42 degrees. In this case, as shown, the light is lost fromthe TIR element 1410 as shown with ray 1433. Hence, while the PVenhancement films with TIR structures 1410 of the invention may enhanceefficiency of a solar cell or PV device in receiving available sunlight,the TIR structures 1410 attempt to enhance efficiencies by increasinglight capture and are designed to increase light capture for particularuses and/or solar array or cell designs (e.g., a differing TIR structuremay be used for a solar array that tracks the Sun than for a solar arraywith no tracking).

One method of testing or verifying a design of a PV enhancement filmand/or a TIR structure design is to plot ray or light intensity receivedby or striking a solar cell's light-receiving surface with and withoutuse of the PV enhancement film. FIG. 15 illustrates a graph 1500 of theray intensity received by a PV or light-receiving surface (such assurface 1411 of FIG. 14) as a function of the angle of incidence. Inthis plot, a TIR element similar to the element 1410 was modeled, but,in this case, the pitch or base width was about 10 mils while the heightor thickness of the TIR element body (as measured from the peak to themating surface of the substrate) was about 2 mils. Line 1510 provides areference for designing a TIR element and indicates the intensity oflight received by a light-receiving surface of a solar cell with nooptical enhancement film and no TIR elements provided, which is thecosine function over the range of angles considered. The plotted line1520 shows the ray intensity received by the same PV light-receivingsurface when a PV enhancement film with sawtooth (or two-faceted) TIRstructures (such as shown in FIG. 4 and FIG. 14) are positioned over oradjacent the light-receiving surface.

For plot 1520, the light-receiving surface (such as surface 1411) wasconsidered to have 100 percent reflection, which is not the case for PVdevices but is useful for determining the functionality of a TIRstructure at trapping reflected rays. As shown with line 1520, the TIRrays or redirected rays reflected from the light-receiving surfaceenhance the intensity of the light-receiving surface over theconventional, uncovered PV device. Use of the TIR structure with a solarcell can be seen to significantly enhance intensity when light isreceived at incidence angles greater than 20 degrees and less than −20degrees (with a little loss or decrease typically occurring in thecentral region due to the approximately 4 percent Fresnel reflectionlosses associated with rays entering the TIR structure). Calculationsshow that the average intensity of rays with the use of the TIRstructure is 0.937 while the reference intensity without a PVenhancement film is 0.699. Hence, an improvement or enhancement gain ofover 30 percent (e.g., about 34 percent) is achieved with this TIRstructure design.

Of course, a typical solar cell or PV device will not have 100 percentreflection from the light-receiving surface as this would not generateelectricity. FIG. 16 illustrates a graph 1600 similar to that shown inFIG. 15 for the same TIR structure but with a light-receiving surfacewith 50 percent reflection. Reference intensity is shown with line 1610for the surface with no TIR structures while line 1620 shows intensityachieved with reflected light being captured or trapped by TIR andredirected back to the light-receiving surface (such as surface 1411 thefacets/sides 1414, 1416 of TIR structure 1410). Intensity issignificantly increased in the ranges of +/−20 degrees to +/−50 degreesby this TIR structure, with calculations showing an overall gain of atleast about 5 percent to 0.735 over an uncovered PV light-receivingsurface. Similar modeling at 75 percent reflection from thelight-receiving surface showed an increase in intensity of about 15percent with use of a TIR structure.

FIG. 17 illustrates a ray tracing 1700 for one embodiment of a PVenhancement film 1710 utilizing a sawtooth arrangement of linear orelongated, two-sided TIR elements 1712, 1720, 1730 (with only 3 shownfor simplicity whereas a typical PV enhancement film will includenumerous TIR structures across one surface). In FIG. 17, the film 1710is shown without a substrate at the base of the elements 1712, 1720,1730 (e.g., a film of plastic, glass, or ceramic or the like) for easeof illustration, and the ray tracing assumed that the light-receivingsurface at the base of the elements 1712, 1720, 1730 had 95 percentreflection. The rays are shown as having an angle of incidence of about−60 to −70 degrees, and it at such larger angles of incidence where theeffects of refraction may become more important.

Specifically, the ray tracing 1700 shows that received ororiginally-incident sunlight or rays 1702 may strike one a firstfacet/side 1714 of a TIR element 1712 (such as at an angle of incidencelarger than about +/−60 degrees or the like) and enter the TIR element1714 where it may be deflected downward to the light-receiving surface1718. Some of the light/rays though may pass through the secondfacet/side 1716 as shown at 1704 to strike an adjacent TIR element 1720upon facet/side 1722. Again, some of the light may be directed withoriginally incident rays 1702 striking facet/side 1722 ontolight-receiving surface 1728. Other portions may pass through facet/side1724 as shown to strike facet/side 1732 of the next TIR element 1730,where it again may be directed to the light-receiving surface 1738 witha portion of the captured/trapped light 1702 or pass through facet/side1734 as unabsorbed light. Light reflected from the surface(s) 1718,1728, 1738 likewise may be directed to adjacent TIR elements 1712, 1720,and/or 1730 where it may pass through or at least in part be directed tothe light-receiving surfaces 1718, 1728, 1738. In this manner, the TIRelements 1712, 1720, 1730 use refraction to recapture reflected rays ortransmitted rays to increase the efficiency by directing portions oforiginally-incident but not absorbed light onto the light-receivingsurface (e.g., a surface of a solar cell or the like for absorption).This refractive collection or trapping of light may be consideredadditive to the overall efficiency gain provided by use of TIR elements(e.g., added to the increased intensity shown in prior figures). Inother words, rays reflected and escaping one TIR element may be capturedby adjacent structures where they can be again directed to thelight-receiving surface.

FIG. 18 illustrates a graph 1800 of light intensity as a function ofangle of incidence of light upon a light-receiving surface (such assolar cell position underneath the base of the TIR elements 1712, 1720,1730). In this modeling/evaluation, the TIR structures were elongated orlinear body structures with a two-sided (or triangular) cross section asshown in FIG. 17 with a base width or pitch of about 10 mils and aheight (or thickness) of about 7 mils. FIG. 18 shows a reference orbaseline intensity with line 1810 of a light-receiving surface without aPV enhancement film of TIR structures, and line 1820 illustrates lightintensity upon this same surface (such as surfaces adjacent the bases1718, 1728, 1738) after positioning a PV enhancement film with TIRstructures (such as film 1710 with structures 1712, 1720, 1730). Theintensity graph 1800 is considering intensity contributions due torefraction effects/capturing, and it shows that the TIR structures of aPV enhancement film may be used to significantly increase intensities athighly oblique angles where losses are often highest for a conventionalsolar cell (e.g., cells in a solar array with no tracking).Specifically, peak intensity gains are seen at angles greater than about+/−40 degrees (with a peak from about +/−60 to 70 degrees). As shown inFIG. 17, the intensity is beneficially increased for angles beyond 60degrees. Note, in some applications a small decrease in the centerportions between +/−40 degrees (as shown for this TIR structure) mightbe acceptable in order to achieve the increase at around 60 degrees ofray incidence.

All or a significant portion of the code or pseudocode used by theinventors in modeling and/or designing the TIR structures/elements forembodiments of PV enhancement films in accordance with the invention isprovided in a program listing after this detailed description, and it isbelieved that this code/program listing will be useful to those skilledin the art in selecting a desirable TIR element for use with a varietyof solar cells and solar arrays. The computer code was used in part tocalculate the curves provided in the attached figures to determineeffectiveness of TIR structures in increasing intensity upon alight-receiving surface over ranges of angles of incidence for receivedlight. In use, after the parameters are read in off a user menu, the PVray tracing routine is called. The computer program that is used todesign and evaluate structures is a non-sequential ray tracing programdesigned to investigate optical structures for PV output powerenhancement. The user enters values on a menu that are to beinvestigated, and these values such as type of structure (e.g.,sawtooth, sinusoidal, cylindrical, and so on) and values for thickness,period, amplitude, and radius (if and as applicable). Deep inside thesubroutine, the Fresnel reflection and transmissions are calculated inorder to find the final intensity of rays striking the PV surface. ThePV trace routine is called and is given in the included code/pseudocode.

The PV trace routine or program will give the user the best values foundin the range of search for the TIR structure evaluated. These bestvalues are entered into the data menu and rays are traced to evaluatethe structure. Typically, 1000 rays are traced for every 2 degrees ofangle between the limits of −80 degrees to +80 degrees, and the resultsplotted as shown in some of the above figures. To examine the rays andhow they are refracted and reflected in the structures a small number ofrays, e.g. 100 rays, might be used at a specific angle of interest asshown in FIG. 17.

The inventors also modeled the PV enhancement film 732 shown in FIG. 7.Ray tracings (not shown) indicated a significant portion of reflectedlight from the PV or light-receiving surface 725 was directed back ontothe surface 725 for second, third, and additional chances forabsorption. FIG. 19 illustrates a graph 1900 showing with line 1910reference light intensity relative to angle of incidence on the surface725 prior to application or installation of the PV enhancement film 730in the assembly 700. Line 1920 illustrates increased intensity acrossthe entire range of angles of incidence for the received light for theassembly 700 after application of the PV enhancement film 730. In thismodeling, the TIR structures 734 had base widths of 10 mils, a thicknessor height of 2 mils (as measured from the substrate 732, and effects ofthe substrate 732 were neglected for modeling purposes), and eachstructure had a wall or light-receiving/trapping facet or side having a7 micron radius (in its arcuate cross section formed by a planeorthogonal to the longitudinal axis of the elongated body of the TIRstructure). The average intensity with the film was found to be 0.875while without the film the average intensity was 0.699, which representsan increase of about 25 percent (with a highly or perfectly reflectivesurface 725, and typical PV devices likely would see smallerimprovements in intensity and corresponding efficiencies).

With the above teaching in mind, one skilled in the art will readily beable to expand the concepts to arrive at numerous additional PVenhancement film and TIR structure designs to provide a particularoptical effect (e.g., particular light trapping for a plannedimplementation or use). For example, FIG. 20 illustrates a solar arrayassembly 2000 that includes a solar array 2010 including a number ofsolar cells 2012, 2014, 2016. The solar array 2010 (or the solar cells2012, 2014, 2016) have a light-receiving surface 2020 that generallywould be positioned to be directed toward or facing the Sun to receivesunlight when the assembly is in use to generate electricity fromavailable solar energy. As discussed above, the design of the solarcells 2012, 2014, 2016 will typically result in the light-receivingsurface(s) 2020 reflecting a percentage of the sunlight that strikes thesurface 2020, with the percentage of reflected or lost (not absorbed)light will vary with the cell design and also upon the angle ofincidence of the sunlight on the surface 2020.

To enhance light capture, the assembly 2000 includes a PV enhancementfilm 2030 that is applied over the solar cells 2012, 2014, 2016 of thesolar array 2010. The film 2030 includes a substrate or sheet 2032 ofclear plastic, glass, or ceramic that is substantially transparent tolight (e.g., highly transmissive) and includes a planar surface that ispositioned adjacent and/or abutting light-receiving surface 2020. Uponan opposite side of the substrate 2032, a plurality of TIR elements 2034are provided, and the elements 2034 have elongated bodies extending intothe plane of the page of the drawing (e.g., FIG. 20 is an end view offilm 2030) with the cross sectional shape of the bodies defined by alight-receiving wall or facet/side 2035. The surface 2035 in thisembodiment is a sine wave (or is sinusoidal) with peaks and valleys.

Modeling of an embodiment of the assembly 2000 provided the graph 2100of FIG. 21 of light intensity as a function of angle of incidence. Theline 2110 models light intensity on the solar array 2010 (or thelight-receiving surface 2020) without the film 2030, and line 2120graphs the light intensity of the light-receiving surface 2020 of thearray 2010 (or individual cells 2012, 2014, 2016) when the film 2030 isincluded in the array 2000 as shown in FIG. 20. To facilitate modeling,the surface 2020 was assumed to be a mirror with 100 percent reflection,and the TIR structures 2034 were assumed to have base widths or pitch ofabout 10 mils and radius/height of about 7 mils. The average intensityas shown by line 2120 with the film 2030 was found to be 0.829 ascompared to a 0.699 reference cosine average intensity without the film2030, representing an increase of about 18 percent with an increase seenacross the entire range of incidence angles.

FIG. 22 illustrates a solar cell assembly 2200 in accordance withanother embodiment of the invention. In this embodiment, the assembly2200 includes a solar cell or other PV device 2210 with alight-receiving surface/layer 2214 (e.g., a Si wafer with a frontcontact and AR coating and/or other layers such as a protective glasslayer). Over the light-receiving surface 2214 of the cell 2210, a PVenhancement film 2230 is provided (e.g., applied by adhesive (not shown)or other methods to the cell 2210) with a substrate 2234 having a planarsurface abutting or proximate to the light-receiving surface 2214. Thisembodiment may be thought of as a cusps-up embodiment, as each TIRstructure 2232 in the film 2230 is defined by an arcuate side/facet2236, which has a radius but is downward curved (e.g., generallyU-shaped) rather than being upward curved as shown in FIG. 7. In onemodeled embodiment, each facet or side 2236 had a radius of about 7 milsand a width/pitch (as measured between peaks of the light-receivingsurface 2236 of film 2230) of about 10 mils. When a light intensity wascalculated for one embodiment of the film 2230, it was found that theaverage intensity was about 0.820 while reference cosine averageintensity was 0.689 (for a 95 percent reflective surface 2214), for again of about 19 percent. In another embodiment, the TIR structures hada pitch of 10 mils, a thickness at the peaks of 3 mils (as measured toinclude the substrate thickness), and cusp to PV material orlight-receiving surface distance of 1 mil. In this embodiment, averageintensity achieved was better at 0.937, which represents a gain (over nofilm) of about 35 percent.

Hence, it will be understood that the particular configuration of theTIR structures may be varied widely to achieve TIR/refraction-basedtrapping of light and to provide improved efficiency for a solar cell orsolar array of cells. The particular TIR element design used may dependupon a number of factors including the material used for the film,manufacturing issues such as costs, and planned use (e.g., for atracking or non-tracking array, to enhance light trapping at particularvalues of angles of incidence, and so on).

In many PV enhancement films, it may be useful to use a single TIRstructure or element design. This may facilitate manufacturing andprovide a consistent optical enhancement or light trapping level acrossthe surface area of the film. However, the inventors recognize thatthere may be a need or desire to use two or more TIR structures (or TIRelement designs) on a single PV film or in an assembly of PV films(e.g., films with differing TIR elements may be used together on asingle solar array or cell to achieve a desired light-trapping effect).For example, a PV enhancement film may be provided with a sawtoothpattern of TIR elements on one side, and the TIR elements may have thesame or differing pitches/base widths and/or the same or differingheights/body thicknesses to practice the invention.

In one film embodiment, the film includes a sawtooth pattern of TIRstructures with a number of TIR structures of a first thickness and anumber of TIR structures with a second thickness that differs from thefirst thickness (but with the same pitch in this case while otherembodiments may differ the pitch for the two TIR structure designs). Thetwo TIR structure embodiments may be alternated on the film or one TIRstructure thickness may be provided for a first set of TIR structuresand then the other TIR structure thickness may be provided for a secondset of TIR structures and so on in this pattern (e.g., 10 TIR structuresof the first thickness and then 10 TIR structures of the secondthickness or another alternating pattern). In this manner, two lightintensity or light-trapping characteristic curves may (or TIR structurefunctionalities) may be blended within a single PV enhancement film (ortwo or more films arranged upon a solar array to provide the differingTIR structures). For example, sheets or films each containing aparticular TIR structure arrangement may be laid upon different portionsof a solar panel of PV cells. Each location would have the opticalresponse characteristics associated with the overlying PV enhancementfilm and its TIR structures. The overall effect from each location tothe output of the panel or array would be an average response that maybe desired by a solar array designer/user. The customer or user of thePV enhancement films could pick and choose the PV enhancement filmsbased on their optical characteristics (e.g., in what incidence angleranges do they provide better performance and so on) and install (in aretrofit example) or provide during manufacture the films across the PVpanel or array to get the optical response they desire. Of course, threeor more TIR structure designs may be used together to enhance theefficiency of a solar panel/array or a solar cell and fine tune a PVresponse.

FIG. 23 illustrates schematically an end view of a solar array (or PVdevice) assembly 2300 that makes use of the concept of two or more TIRstructure designs being included in a single solar array assembly oreven a single PV enhancement film. As shown, the solar array 2310, whichtypically would be formed with a plurality of solar cells, has a lightreceiving surface/layer 2312 that is positioned in use to receivingincident sunlight. To trap light reflected or lost from surface 2312,the assembly 2300 includes a PV enhancement film 2330 (e.g., arelatively thin sheet of transparent plastic, glass, ceramic, or thelike) that is positioned to at least partially cover the light-receivingsurface 2312 (e.g., front surfaces of solar cells or other portions ofPV structures). Instead of a single TIR structure design, the film 2330includes two TIR structure arrangements that are alternated across thesurface of the film 2330 (whereas other embodiments may include two ormore of one design and then alternate with the second (or more) design).

The film 2330 includes a substrate or film 2332 of substantiallytransparent material, e.g., with a planar surface for abutting or beingpositioned proximate to light-receiving surface 2312. The film 2330further includes a plurality of TIR structures upon the substrate 2332with alternating configurations. As shown, a first configuration isrepresented by TIR structure 2340 that includes first and second facets2342, 2344 (e.g., has a triangular cross section), and a secondconfiguration is represented by TIR structure 2350 that includes firstand second facets 2352, 2354 (e.g., has a triangular cross section). Inthis embodiment, the TIR structures 2340 and 2350 have the same basesize or pitch but differing heights/thicknesses to provide differingoptical characteristics (e.g., to achieve differing reflectivity or TIReffects at varying angles of incidence of sunlight upon the solar array2310). These TIR configurations are alternated across the width orlength of the light-receiving surface of the film 2330. While sawtoothdesigns are shown in FIG. 23, it will be understood that other TIRstructures may be mixed and/or alternated such as structures with 3 ormore facets (e.g., to provide a triangular cross section, a pyramidcross section with three, four, or more facets), arcuate or cylindricalstructures, sinusoidal structures, inversed curve (or cusp-up) designs,and so on.

To evaluate the film 2330 of FIG. 23, the pitches (or base widths) ofthe TIR structures were set at 10 mils while the height or thickness ofthe first TIR structure 2340 was set at 3 mils and the height orthickness of the second TIR structure 2350 was set at 4 mils. FIG. 24illustrates a plot or graph 2400 of light intensity as a function ofangle of incidence on the light-receiving surface 2312 (which wasassumed to be 95 percent reflective or a mirror in this test run).Reference cosine average intensity is shown with line 2410 without useof the film 2330, and average intensity at the surface 2312 with use ofthe alternating sawtooth PV enhancement film 2330 is shown with line2420. When a film with only 3 mil thick TIR structures was modeled usingthe included TIR element evaluation or ray tracing routine, there was arelatively large gap in achieved gain in the smaller values of angle ofincidence. But, when a film with only 4 mil thick TIR structures wassimilarly modeled, there was relatively high gain in these same rangesof values of the incidence angle (e.g., −18 degrees to 18 degrees or thelike). Then, when the film 2330 of FIG. 23 was modeled with alternatingthicknesses of 3 and 4 mils was tested as is shown in FIG. 24, it wasfound that light intensity was enhanced more effectively across therange of incidence angles. The average intensity was found to be 0.901with the film as compared to 0.689 without, for a gain of about 31percent (but, of course, when reflection is much lower than 95 percentthe gain or enhancement achieved will be lower).

The inventors recognize that many other cross sectional and overallshapes for the TIR elements may be useful to practice the invention, andmany of these configurations are believed to be considered within thebreadth of this description when considered alone or in combination withthe attached figures. For example, FIG. 6 illustrates use of linear,three-sided or faceted TIR element 614 in the PV enhancement film 610.These may be considered to have pyramid cross sections with three sides.The TIR elements 614 in some preferred embodiments may be modified toinclude 4 sides/facets such that the two lower sides of the TIR elementare angled inward from the substrate 612 and the upper sides/facets maybe angled inward at a sharper angle to meet at a point/apex. In otherembodiments, five or more facets may be included in the cross section ofthe linear or elongated TIR elements 614. Likewise, such 3, 4, or moresided TIR elements may be used in the sawtooth PV enhancement film 2330shown in FIG. 23 with varying designs being alternated (two, three, ormore cross sectional shapes may be provided in a repeating pattern orprovided in groups that repeat once, twice, or more times across thesurface of the film 2330). Further, the inventors understand that PVenhancement films shown in FIGS. 8-11 may be modified to include pyramidshaped TIR elements (in place of the cone or frustoconical shapedstructures and dome structures or to supplement such pyramid-shaped TIRelements). These standalone pyramid TIR elements may be three, four, ormore sided pyramids in which the sides meet at a point (e.g., with eachside mating with the substrate at a variety of angles such as 30 to 70degrees with 45 to 60 degrees being useful in some embodiments) ormeeting at a peak surface, which may be planar (e.g., a flat top suchthat the shape is a truncated pyramid), domed/arched, or anotherconfiguration facilitating manufacturing and/or capture of receivedsunlight.

It may also be useful to further discuss the decision of whether toprovide TIR elements that are pyramids and/or three-dimensionalstructures or that are linear triangular cross section structures. For aperfect orientation of linear triangular structures (TIR structures) tothe seasonal azimuth of the Sun, the direction of the linear structureswould be in a perfect North to South orientation, and the panel would beplaced in a perfect orientation East and West with the panel facing theSun at mid day perfectly with both axis points parallel to the pathwayof the Sun. The above placement/use, however, can typically only beaccomplished by mounting panels exactly to the azimuth of the sun in theEast to West orientation. However, this is not likely or oftenpractical, as most rooftops are not conducive to this and rather haveoff-angles in most cases. In addition, even if this were possible (withmounting brackets, by design, or with luck), one would still need toadjust for the seasonal azimuth to the sun to compensate for the42-degree angular difference from summer to winter.

The testing procedure and software used to model PV enhancement filmsallowed the additional efficiencies obtained by using linear TIRstructures to be measured and modeled in a linear diagram withtriangular cross section structures (e.g., in order to observe theindividual angular effects and resulting TIR with “ideal” placement),and, hence, some of the initial designs and prototypes utilizedtwo-faceted TIR elements. Given ideal circumstances (e.g., perfectplacement), the results would be measureable with these triangularstructures and results documented appropriately. However, since idealplacement in the field or home use is unlikely, a three-dimensionalstructure such as a pyramid is believed by the inventors to provideperformance enhancement at off-angles in the direction perpendicular tothe linear structure (e.g., a three-faceted pyramid, a four or moresided structure, or similar cross section may be preferred in manyapplications). Overall, performance enhancement characteristics innon-perfect installations (most installations) may be as much as doubleor more using pyramidal or three-dimensional (non-linear or linear or 4or more sided) structures. Typical placement of panels (non-idealplacement) provides not only normal cosine fall off and decreasedefficiency from normal sunrise to sunset but also non-perfect placementto the seasonal azimuth of the Sun resulting in additional cosine falloff and resulting reflective losses. Since a pyramidal structure havingappropriate benefiting angles in several directions increases TIR anddecreases cosine fall off from several angles, overall efficiencies areimproved even more than a control model with perfect placement to theSun. In other words, a non-perfectly placed panel may achieve 10%efficiency overall without the film yet may achieve 18% or more with thepyramidal film, while only 14% may be achieved with linear structuresbecause of the incident angles to the structures.

Numerous methods of manufacturing may be used to provide the PVenhancement films (and apply these films to existing or being fabricatedcells, PV devices, and/or solar arrays). For example, various base filmsmay be used as carriers, and the carrier film itself may or may not havephotovoltaic material already applied to the film. Base films may bePET, acrylic, OPP, Polycarbonate, polyethylene (high or low density) oreven thin glass. The TIR structures may be engraved into a metal, glass,ceramic, rubber, photopolymer or plastic cylinder with diamond tooling,or by a photo-polymeric exposure system, or even laser engraved. Thestructures may be placed into the film with a combination of heat andpressure with an embossing system in a roll form, or with a hot diestamp and platen directly onto the film itself. Conversely, they mayalso be cast with UV cured liquid or e-beam, or any type of energy curedpolymer or solvent based polymer in roll or sheet form. One preferredmethod of manufacture may be electron beam casting using polymers thatare engineered for durability in the sun.

Many of the above examples and evaluations were limited to atwo-dimensional analysis or ray tracing of incident light. In otherwords, the results in the program take into account a linear structurerunning in a North-South direction, with the Sun rising in the East andsetting in the West and with the collector optimized for the seasonalityof the Sun. Therefore, it is believed that implementing the TIRstructures as nonlinear, 3-dimensional shapes such as pyramids, cones,domes, frustoconical shapes, and the like may provide even betterresults and potentially increase the effectiveness in a real lifeapplications exponentially. Keeping this in mind, overall increases inefficiency may go up to more than 100 percent in many cases dependingupon the placement of the solar panels or PV devices with the PVenhancement films (e.g., which may be determined by roof shape not basedupon optimum angular placement in residential settings and somecommercial applications).

In brief, the PV enhancement films provide structures over a solar cell,a solar array, a PV film, or combined with PV film or materials, and theTIR structures are adapted to purposely create total internalreflection. The PV enhancement films (or at least the TIR structures)are made of glass, plastic, ceramics, or energy cured polymers that aresubstantially transparent (e.g., transmit a large portion of receivedlight). The TIR structures may be nearly any size or thickness, but, inpractice due to economic realities, manufacturing challenges, and otherreasons, the TIR structures (and the films) often will be under about100 mils in thickness and routinely under about 5 mils. Some embodimentsof the PV enhancement films include elongated or linear TIR structureswith two, three, or more straight (or near straight) facet or sidesangling inward from the substrate of the film at 10 to 60 degrees. Thesestructures may be saw tooth shapes (when viewed in cross or on end) with50 degree (45 degrees may be ideal for directly overhead sun) to under20 degree inwardly-angled sidewalls. In other embodiments, PVenhancement films may include 3D or non-elongated TIR structures thatcan be 3-sided to a sharp point, be hexagonal shapes to a sharp point,may be cone shaped structures 10 to 60 degrees sidewalls with a point ora flat, planar top, may be circular in cross section to form a series ofdomes on the PV film surface, or have other shapes to provide TIReffects in a 3D context. It should be noted that the TIR structures,including the linear lenticular-like structures with regular Pi or roundradii cross sections designed to go over the top of the PV materials, donot act to “focus” the received or incident light but act to trapreflected light coming off of the covered solar cell or light-receivingsurface (e.g., the lenticules of such a PV enhancement film aretruncated and appear to be parts of “circles” from a side view and arenot configured for focusing received light onto the solar cell). The 3DTIR structures may be dome shaped or “fly's eye” structures. Again, theTIR structures are not designed to focus light (and do not work likeregular lenses), but, rather, the TIR domes sit directly on the PVmaterials (or are the base film for the PV materials) and act to provideTIR trapping of reflected light from the PV materials. A normal lenswould have to have a height-to-width ratio of about 1.5 or 2 to 1H/W tocreate a focus in a linear or round lens. In contrast, the described TIRstructures almost have no height (e.g., are very thin in most cases) andmay be thought of as truncated “circles” or thin hemispheres sitting orpositioned upon the light-receiving surface of the PV material or of thesolar cell(s).

One way to understand some of the concepts of this invention is tounderstand that a certain percentage of rays that enter through themicrostructures (e.g., TIR elements or structures) and hit the filmsubstrate and then the PV material (or light-receiving surface of asolar cell/array) will always be lost. This is due to the relationshipof the exiting angles and the point of impact of the wall and thoseangles (e.g., reflect in directions such that they strike the sides orfacets of the TIR structures/elements at angles greater than 42degrees). It is important to understand that the percentage of deflectedrays from PV materials is much greater when the angle of the Sun inrelationship to the panel (which includes the PV materials such aswithin solar cells) is at a greater angle, which leads to greaterdeflection and less efficiency, and this occurs typically when the Sunis not at perfect angles to the collector (e.g., not directly overheadat noon or the like). However, a great many of the unabsorbed rays thatare deflected from the PV film will try unsuccessfully to exit themicrostructures and will be deflected through TIR one or more times(e.g., to a theoretical but nearly impossible infinite number of times),and this TIR functionality of the TIR elements of the invention createsmultiple chances and multiple possibilities (and practical differencesin angles at every “strike”). Overall, the TIR structures provide asignificant mathematical increase in probability for absorption. Forinstance, operation of a solar array may experience 50 percent of therays are being lost due to cosine fall off or other structuraldeflections during the daytime as the Sun moves across the sky. But,with the use of the PV enhancement films designed in accordance with oneor more embodiments of the invention with one or more TIR structureconfiguration, most of the deflections are eligible for TIR deflectionback to the PV materials (or light-receiving surfaces of a solar arrayor solar cell or PV device).

The following formulas may be used, with actual data from PV materialsfor input, to test individual PV enhancement films and/or PV assembliesthat include such films and TIR structures. A possible example is asfollows:

X=deflected rays (70% of rays)

Z=newly absorbed rays from TIR (20% of X)

S=original efficiency (100%-70%=30% and then (0.30)(0.20)=0.06 or 6%)

H=increased efficiency: with X=70%, H=(0.20)(X)=(0.20)(0.70)=0.14

Total efficiency=S+H or, in this example, 6%+14%=20% (versus 6 percentoriginal)

More importantly, the above was drawn from the following formula, eachindividual range below in angles represents an efficiency gain at eachof the angles including cosine falloff as a result of the followingangles times 2 (covering 140 degrees) as a result of a positive angle ora negative angle.

I=incoming rays from 60 to 70 degrees (also −60 to 70 degrees)

J=incoming rays from 50 to 60 degrees (also −50 to −60 degrees)

K=incoming rays from 40 to 50 degrees (also −40 to −50 degrees)

L=incoming rays from 30 to 40 degrees (also −30 to 40 degrees)

M=incoming rays from 20 to 30 degrees (also −20 to −30 degrees)

N=incoming rays from 10 to 20 degrees (also −10 to −20 degrees)

O=incoming rays from 0 to 10 degrees (also −10 to 0 degrees)

The above represents approximately 12 hours of daylight or sunshine.Negative numbers represent morning and positive numbers representafternoon. Positive and negative numbers will yield the same data. Forpurposes of formula, each set of angles will count twice (one fornegative and one for positive). Currently, this formula does not takeinto account angles resulting from other than ideal panel placement in anorth south seasonal azimuth. The reason for this lies in the existingprogram, and the method of ray racing the program in a linear way and intwo dimensions instead of three dimensions. This can be addressed laterin future program versions. In other words, it is important to note thatin these conditions further mathematical efficiency improvements wouldbe found from this invention. Additionally, linear TIR structures maynot yield the best results, and this is why several different TIRstructures are and PV enhancement films are shown in the figures withnon-linear or non-elongated TIR structures.

Therefore, to find the values of each value (I-O) use, V=Value(indicating total value) times 2 (not squared). For example: Total valueof I=value of I×2, which takes into account the morning and eveningvalues. For the purpose of this modeling, the rays being deflected inthe ray tracing program striking the strips of absorbers will simulateabsorption of the ray, when in fact that ray may be deflected again bythe PV material, and then be sent back again by the microstructure.Since the inventors found it difficult to perform this rather subjectiveexperiment with the ray tracing program, the following procedure wasused to evaluate TIR structures and PV enhancement films:

Baseline

1. 50% of the “space” between the measurements of the proposed (beforeplacing the structures) structures has 10 separate strips equating to 5%each of the total space as absorbers, and the corresponding remainderare mirrors.

2. Rays are then traced at each value (average value) in each categoryby changing the incoming ray angles in the programs.

3. Absorbed rays are then recorded with the same input data.

Micro Structures Addition

1. Repeat the above, only with the structures.

2. Measure the values using the structures, and calculate the increasedabsorption.

3. Calculate the absorption by value (angle)

4. Calculating the theoretical addition of the addition of variance inthe “Y” axis describing efficiency increases in normally placed fieldapplication (imperfect placement).

1. A solar cell assembly for more efficiently capturing solar energy,comprising: a solar cell including a light-receiving surface, wherein afraction of light incident upon the light-receiving surface is reflectedover a predefined range of incidence angles; and a photovoltaic (PV)enhancement film of substantially transparent material covering at leasta portion of the light-receiving surface, the PV enhancement filmcomprising a substrate positioned proximate to the light-receivingsurface and comprising a plurality of total internal reflection (TIR)elements on the substrate, wherein the TIR elements direct at least aportion of the reflected light to strike the light-receiving surface ofthe solar cell, wherein each of the TIR elements comprises an elongatedbody extending along the substrate including at least two sides angledinward with adjacent ones of the bodies having spaced apart facets thatdefine a space substantially free of the substantially transparentmaterial of the PV film, wherein the bodies of the TIR elements have atleast two differing symmetric triangular cross sectional shapes with twosides, wherein the cross sections are taken along a plane passingorthogonal to a longitudinal axis, wherein first ones of the differingcross sectional shapes each have a height of less than about 3 mils andsecond ones of the differing cross sectional shapes each have a heightgreater than the height of the first ones of the differing crosssectional shapes and less than about 4 mils, and wherein the at leasttwo differing cross sectional shapes each comprise first and secondsides extending inward to meet at a peak surface and wherein the firstand second sides extend inward at angles selected from the range of 45to 60 degrees.
 2. The solar cell assembly of claim 1, wherein the TIRelements transmit the light incident upon the light-receiving surface tothe solar cell and direct the portion of the reflected light using TIRas the reflected light strikes one or more sides of at least one of theTIR elements.
 3. The solar cell assembly of claim 1, wherein adjacentones of the TIR elements have differing heights as measured from thesubstrate to a peak of the triangular cross section of the TIR elements.4. The solar cell assembly of claim 1, wherein each of the TIR elementshas a base and a height less than about 10 mils in magnitude.
 5. Aphotovoltaic apparatus, comprising: a layer of photovoltaic materialwith a light-receiving surface; and a plurality of total internalreflection (TIR) elements positioned adjacent to the light-receivingsurface, wherein each of the TIR elements comprises a body formed ofsubstantially transparent material, the body being shaped to transmitlight incident upon an outer surface of the body toward thelight-receiving surface and to direct at least a portion of lightreflected from the light-receiving surface back toward thelight-receiving surface, wherein the TIR elements comprise a pluralityof elongated, side-by-side members and wherein the bodies face away fromthe photovoltaic material and have triangular cross sections having atleast two differing cross sectional shapes, wherein the cross sectionsare taken along a plane passing orthogonal to a longitudinal axis,wherein first ones of the differing cross sectional shapes each have aheight of less than about 3 mils and second ones of the differing crosssectional shapes each have a height greater than the height of the firstones of the differing cross sectional shapes and less than about 4 mils,and wherein the at least two differing cross sectional shapes eachcomprise first and second sides extending inward to meet at a peaksurface and wherein the first and second sides extend inward at anglesselected from the range of 45 to 60 degrees.
 6. The apparatus of claim5, further comprising a sheet of substantially transparent materialpositioned with a first planar side proximate to the light-receivingsurface and a second planar side supporting the TIR elements.
 7. Theapparatus of claim 5, wherein substantially all of the reflected lightfrom the light-receiving surface that strikes a surface of one of theTIR elements at an angle of less than about 42 degrees is reflected as apart of the portion directed back to the light-receiving surface.
 8. Asolar array, comprising: a plurality of solar cells arranged in a planarpanel with a light-receiving surface; and a photovoltaic (PV)enhancement film comprising a substrate with a planar first surfaceproximate to the light-receiving surface and a second surface comprisinga plurality of light-trapping structures, the PV enhancement film beingformed of a substantially transparent material wherein each of thelight-trapping structures comprises at least one facet receivingincident light, directing the incident light toward the light-receivingsurface of one of the solar cells, and reflecting a portion of lightreflected from the light-receiving surface, the reflecting beingperformed using total internal reflection (TIR), wherein thelight-trapping structures comprise a plurality of elongated and parallelmembers each having at least two differing symmetric triangular crosssectional shapes with two sides, wherein the cross sections are takenalong a plane passing orthogonal to a longitudinal axis, wherein firstones of the differing cross sectional shapes each have a height of lessthan about 3 mils and second ones of the differing cross sectionalshapes each have a height greater than the height of the first ones ofthe differing cross sectional shapes and less than about 4 mils, andwherein the at least two differing cross sectional shapes each comprisefirst and second sides extending inward to meet at a peak surface andwherein the first and second sides extend inward at angles selected fromthe range of 45 to 60 degrees.
 9. The solar array of claim 8, whereinthe elongated members comprise at least two facets for the receiving andthe directing of the incident light and for the reflecting of theportion of the reflected light.
 10. A solar cell assembly for moreefficiently capturing solar energy, comprising: a solar cell including alight-receiving surface, wherein a fraction of light incident upon thelight-receiving surface is reflected over a predefined range ofincidence angles; and a photovoltaic (PV) enhancement film ofsubstantially transparent material covering at least a portion of thelight-receiving surface, the PV enhancement film comprising a substratepositioned proximate to the light-receiving surface and comprising aplurality of total internal reflection (TIR) elements on the substrate,wherein the TIR elements direct at least a portion of the reflectedlight to strike the light-receiving surface of the solar cell, whereineach of the TIR elements comprises an elongated body including at leasttwo sides angled inward and facing away from the light-receiving surfaceof the solar cell and wherein each of the TIR elements comprises threeinterconnected sides including two sides angled inward and an upperplanar side disposed between the two angled sides that is substantiallyparallel to the light-receiving surface, wherein the bodies of the TIRelements have at least two differing symmetric triangular crosssectional shapes with two sides, wherein the cross sections are takenalong a plane passing orthogonal to a longitudinal axis, wherein firstones of the differing cross sectional shapes each have a height of lessthan about 3 mils and second ones of the differing cross sectionalshapes each have a height greater than the height of the first ones ofthe differing cross sectional shapes and less than about 4 mils, andwherein the at least two differing cross sectional shapes each comprisefirst and second sides extending inward to meet at a peak surface andwherein the first and second sides extend inward at angles selected fromthe range of 45 to 60 degrees.
 11. A photovoltaic apparatus, comprising:a layer of photovoltaic material with a light-receiving surface; aplurality of total internal reflection (TIR) elements positionedadjacent to the light-receiving surface, wherein each of the TIRelements comprises a body formed of substantially transparent material,the body being shaped to transmit light incident upon an outer surfaceof the body toward the light-receiving surface and to direct at least aportion of light reflected from the light-receiving surface back towardthe light-receiving surface; and a sheet of substantially transparentmaterial positioned with a first planar side proximate to thelight-receiving surface and a second planar side supporting the TIRelements, wherein the bodies of the TIR elements are elongated with atleast two differing symmetric triangular cross sectional shapes with twosides, wherein the cross sections are taken along a plane passingorthogonal to a longitudinal axis, with the two sides defining a lightreceiving and trapping surface using TIR to direct the portion reflectedlight back to the light-receiving surface, wherein the TIR elements onthe light receiving and trapping surface face away from the layer ofphotovoltaic material, wherein the TIR elements each have a base widthand a height of less than about 10 mils, wherein first ones of thediffering cross sectional shapes each have a height of less than about 3mils and second ones of the differing cross sectional shapes each have aheight greater than the height of the first ones of the differing crosssectional shapes and less than about 4 mils, and wherein the at leasttwo differing cross sectional shapes each comprise first and secondsides extending inward to meet at a peak surface and wherein the firstand second sides extend inward at angles selected from the range of 45to 60 degrees.
 12. The apparatus of claim 11, wherein the first andsecond sides of the first ones of the differing cross sectional shapesextend inward at a first angle in the range of 45 to 60 degrees and thefirst and second sides of the second ones of the differing crosssectional shapes extend inward at a second angle in the range of 45 to60 degrees that is greater than the first angle.